Data bus systems are used in many diverse technical areas. Examples are automotive and aviation engineering, where particularly the communication of electronic control units (ECU) is realized using bus systems, such as CAN bus. If messages are sent from one bus subscriber to all further bus subscribers in a bus system, this is referred to as what is known as a broadcast method. If this involves the data bus operating without defined time windows in which a particular bus subscriber can send a message, the sending processes of the bus subscribers are effected at arbitrary instants, but particularly when the data bus is not being used by a transmission process that is already taking place. In the case of a CAN data bus, the messages are transmitted in the form of data frames. In order to handle the growing communication with an extended bandwidth that accompanies an increasing number of applications, the CAN2.0B protocol was taken as a basis for developing the protocol CAN-FD (“CAN with Flexible Data Rate” specification version 1.0 dated 17 Apr. 2012), which is incorporated by reference, which allows higher data rates and more useful data per data frame. The precise structure of a CAN-FD data frame can likewise be gathered from this specification.
In this case, CAN-FD shares the same physical level with the CAN2.0B protocol, but the formats of the data frames are different. A CAN-FD data frame additionally comprises two control bits, the first control bit allowing data frames of extended length and the second control bit allowing changeover to a faster data rate. The control bits can be used to prescribe both properties independently of one another. CAN-FD can therefore be used to transmit a greater volume of data per unit time (payload). CAN-FD data frames are distinguished in that during the arbitration phase they have the same bit rate as data frames based on the CAN2.0B protocol. In the control field that adjoins the arbitration field of a data frame, the FDF bit (old name “r0”) is used as an indication that a CAN-FD data frame is present. In the Bosch CAN-FD specification, the bit is denoted by EDL, which has not been developed further, however, and leads to IS011898-1, which currently forms the “Draft International Standard” If a CAN-FD data frame is involved, the FDE′ bit is followed by a reserved bit (“res”) and then by the BRS bit, which, on the basis of its value, prescribes changeover of the data rate within the CAN-FD data frame from the bit rate of the arbitration phase to the correspondingly preconfigured and higher bit rate of the data phase. This changeover would be effected as soon as the BRS bit appears. In this case, the definition in the specification is that a recessively detected BRS bit signals the changeover to the faster data rate.
CAN-FD is backward compatible with CAN2.0B, which means that data frames in CAN2.0B format and data frames in CAN-FD format can coexist in the same network. Provided that the CAN-data format is not being used, CAN2.0B data frames can be used in CAN-FD implementations.
CAN transceivers frequently need to meet defined demands on spurious emission, particularly on line-conducted radiated emission. For this purpose, the greatest possible mirror symmetry for the complementary levels on the CAN-H and CAN-L lines and rise times matched to the admissible radiated emission are implemented for transitions between dominant and recessive phases, the shortest possible rise time for the edges not being suitable for every application. For the transmission of CAN2.0B data frames, whose bit time (period for transmitting a bit) is limited to a minimum length of 1 μs, decoding of the bits is usually possible even in the case of a relatively shallow edge profile (relatively long rise time). In the case of decreasing bit times, for essentially constant rise and fall times, the bit pulses are rounded, since the rising and falling edges coincide. With very short bit times, it would not be possible for corresponding data frames to be decoded correctly, which means that despite an increase in the radiated emission the rise and fall times need to be chosen such that decoding is possible reliably.
If a receiving bus subscriber or transceiver recognizes erroneous transmission of a data frame, it sends an error message. This overwrites any communication on the CAN bus, with every further receiving bus subscriber recognizing the error message and rejecting the data frame received up to that instant. The sending bus subscriber terminates transmission as a result of recognition of the error message. Usually, every bus subscriber has a reception error counter (REC) that is incremented when an error message is sent. When a threshold value for a maximum number of error messages sent is exceeded, the bus subscriber sending said error messages is automatically isolated from the data bus. In addition, every bus subscriber comprises a transmission error counter (TEC) for failed transmissions, which is incremented when an error message is received while a data frame is being sent by the sending bus subscriber. Once the TEC has exceeded a previously defined threshold value for a maximum number of transmission attempts, the bus subscriber automatically isolates itself from the data bus, which prevents further transmission attempts. The error counters are decremented for messages sent or received without error.
CAN transceivers that are designed for partial networking (PN) comprise a decoder that can be used to concomitantly read and to decode the arriving CAN data frames in low-energy mode. At present, the specified requirement for decoding is limited to CAN2.0B data frames—that is to say not CAN-FD data frames. In the case of CAN-FD data frames, the bit times in the data phase may be so short that they cannot reliably be decoded correctly by the decoder of the bus subscribers (receivers) with that edge setting of the transmitter that is optimized for radiated emission. In this way, syntactically correct CAN-FD data frames sometimes incorrectly result in recognition of a supposedly erroneous data frame, sending of an error message and incrementing of the reception error counter (REC). This can additionally restrict the availability of affected systems if the error counter reaches an appropriate limit value.